Evidence from biometric and molecular studies indicates that genetic factors significantly influence body mass among humans. This has led to recent efforts to identify single nucleotide polymorphisms (SNPs) across to the entire human genome that are associated with adult and adolescent weight. However, all existing genome wide association (GWA) studies of body mass and obesity have focused on main genetic effects rather than interaction effects between genetic and environmental factors. This leads to gene-environment interaction (GxE) studies that focus on environmental factors that moderate genetic main effects. In the event that there are only genetic effects within particular environments (e.g., no main genetic effects), then current GWA models will overlook important genetic influences. Because body mass is strongly influenced by social environmental factors and because genetic associations for body mass are contingent upon social- environmental influences, environmental risk (and protective) factors must be included in the conceptual understandings and methodological approaches to GWA. While GxE studies involving a single genetic variant are increasingly common, no existing work has specifically focused on genome-wide approaches to GxE. Our approach is a fundamentally new way of examining genetic influences on body mass that extends established GWA methods and draws upon established GxE theory.